1. Field of the Invention
The present invention relates to a rewritable nonvolatile semiconductor memory device, and particularly relates to a floating-gate nonvolatile semiconductor memory device.
2. Description of the Related Art
Semiconductor devices include rewritable semiconductor memory devices (nonvolatile memories) which are called floating-gate nonvolatile semiconductor memory devices. General floating-gate nonvolatile semiconductor memory devices have memory transistors which are constituted so that a tunnel gate insulating film, a floating gate, an inter-electrode insulating film and a control gate are sequentially stacked to cover a channel area between a source area and a drain area formed on a surface layer of a semiconductor substrate.
In recent years, miniaturization and high-integration of the semiconductor devices are noticeable. In the floating-gate nonvolatile semiconductor memory devices, as scaling down of cell structure proceeds, influence of so-called bird's beak formed at the edge of capacitor insulating films increases. When the bird's beak is formed on the inter-electrode insulating film, a capacitance of an inter-electrode insulating composed of the floating gate, the inter-electrode insulating film and the control gate is reduced. When the capacitance is reduced, a coupling ratio is also reduced so that an electric field applied to the inter-electrode insulating film increases. As a result, reliability and performance of the floating-gate nonvolatile semiconductor memory device as well as reliability and performance of the memory transistor deteriorate.
Such a problem is not much noticeable because even if a bird's beak is formed on a terminal of inter-electrode insulating film, a percentage of the area of the bird's beak with respect to the area of the whole inter-electrode insulating film is small in a generation where a gate width is comparatively large. However, when floating-gate nonvolatile semiconductor memory devices are further miniaturized in the future, the percentage of the area of a bird's beak portion with respect to the area of the whole inter-electrode insulating film increases. For this reason, such a problem is unignorable. Therefore, in the case where the floating-gate nonvolatile semiconductor memory devices are further miniaturized in the future, it is an important problem to prevent a bird's beak. From another point of view, it is an important problem to control an amount of the bird's beak within an allowable range.
Normally, the bird's beak of the inter-electrode insulating film is formed when a sidewall of the memory transistor is subject to an oxidizing process. The oxidizing process to be given to the transistor sidewall is executed in order to recover from plasma damage generated in a reactive ion etching process to form a gate of the memory transistor. Oxidizing species for oxidizing the transistor sidewall diffuse to the inside of the inter-electrode insulating film from the insulating film edges. The bird's beak of the inter-electrode insulating film is formed by oxidation of surfaces of the floating gate and the control gate made of polycrystal silicon.
A method of controlling a bird's beak generated due to such a mechanism includes the following methods. Jpn. Pat. Appln. KOKAI Publication No. 7-249697 proposes a technique of a NONON film structure such that when an inter-electrode insulating film is formed into an ONO film structure composed of three layers: an oxide film, a nitride film and an oxide film, SiN films are provided on both upper and lower faces of the ONO film. Similarly, U.S. Pat. No. 5,661,056, U.S. Pat. No. 5,907,183 and the like propose that a SiN film is inserted between an inter-electrode insulating film and a floating gate electrode or a control gate.
The method of forming the SiN film between an inter-electrode insulating film whose top layer and bottom layer are made of oxide films and upper and lower polycrystal silicon electrodes includes the following methods, for example. In one method, a polycrystal silicon electrode is subject to a thermal nitriding process in an NH3 atmosphere so that a SiN film is deposited on an interface between a floating gate electrode as a lower electrode and the inter-electrode insulating film. In another method, a SiN film is deposited on a polycrystal silicon electrode by a CVD method. In still another method, a SiN film is deposited on an oxide film as the top layer of an inter-electrode insulating film on an interface between a control gate electrode as an upper electrode and the inter-layer insulating film. As a result of examinations by the inventors of the present invention, however, when the SiN film is simply inserted between the inter-electrode insulating film and the floating gate electrode or the control gate electrode, a bird's peak of the inter-electrode insulating film can be prevented, but an operation of a floating-gate nonvolatile semiconductor memory device such as charge retention property is affected.